This invention relates to a process of producing methanol from gaseous hydrocarbons having a lower C/H ratio than is stoichiometrically required to produce methanol, comprising catalytically cracking said hydrocarbons in the presence of water vapor at 830.degree. to 930.degree. C. and 5 to 30 bars to produce a synthesis gas which consists essentially of hydrogen and oxides od carbon, and subsequently effecting a catalytic reaction of hydrogen with oxides of carbon at 230.degree. to 280.degree. C. and 30 to 150 bars.
It is known that methanol can be produced by a process in which light hydrocarbons in contact with a nickel-containing catalyst are cracked at 800.degree. to 900.degree. C. and pressures between 1 bar and about 30 bars in the presence of water vapor to produce a synthesis gas which consists essentially of hydrogen, oxides of carbon, and residual methane and which is cooled with condensation of the residual water vapor and is then compressed to pressures of 40 to 120 bars and contacted with a catalyst that consists substantially of copper at temperatures of 230.degree. to 280.degree. C. whereby methanol is formed from hydrogen and part of the oxides of carbon (H. Hiller, F. Marschner, E. Supp, The LURGI-Low Pressure Methanol-Process, CEEF (Tokyo); chem. Economy and Engineer Review, September 1971).
Methanol can also be produced from a synthesis gas which has been produced by the cracking of hydrocarbons in contact with an indirectly heated, nickel-containing catalyst at temperatures above 700.degree. C. in the presence of water vapor. To produce methanol from said synthesis gas, the latter is contacted at 230.degree. to 280.degree. C. and 30 to 80 kg/cm.sup.2 with a copper-containing catalyst contained in tubes which are indirectly cooled with water. It is known that the heat generated by the methanol-producing reaction can be utilized by extracting it from the reactor tubes and using it to produce high-pressure steam.
At least part of the high-pressure steam thus produced may be expanded against a back pressure of 4 to 6 kg/cm.sup.2 with performance of work and the resulting low-pressure steam may be used as a heat source for the final distillation or the resulting high-pressure steam may be expanded to ambient pressure in condensing turbines (German Pat. Specification No. 2,013,297-U.S. Pat. No. 3,713,784).
In a process in which the heat generated by the methanol-producing reaction is not used to produce water vapor, the waste heat from the tubular heater, i.e., the difference between the supplied heat and the heat required to heat the natural gas-water vapor mixture from the inlet temperature to the reaction temperature and for the endothermic cracking reaction, plus the heat content of the hot cracked gas, is not sufficient to meet the heat and power requirements involved in the production of methanol so that additional fuel must be burnt. On the other Hand, those processes in which the heat generated by the synthesis of methanol is utilized to produce water vapor result in a certain heat and energy surplus, which is not utilized in the process itself because the water vapor produced by the heat of reaction is at a lower temperature than the tubular heater for this reason cannot be used to supply heat to the tubular heater so as to reduce the fuel consumption.
Where natural gas is used as a feedstock, the known processes in which the heat generated by the methanol-producing reaction is not utilized or is utilized only in part involve a heat consumption of 8.1.times.10.sup.6 to 8.4.times.10.sup.6 kcal per ton of pure methanol product.
In processes in which the heat of reaction is virtually completely utilized to produce water vapor, the heat consumption is 7.8.times.10.sup.6 to 7.9.times.10.sup.6 kcal.
It is an object of the invention to further reduce the heat consumption per ton of pure methanol produced, also to provide an optimized and more economical methanol synthesis process, and to reduce the capital requirement.
This object is accomplished according to the invention in that part of the heat required for the catalytic cracking of the hydrocarbons is transferred from the hot synthesis gas to the hydrocarbon-water vapor mixture flowing through the cracking catalyst whereby the consumption of thermal-/energy per unit of methanol product is reduced.
The heat is suitably transferred in a temperature range between a synthesis gas end temperature of at most 930.degree. C. and a hydrocarbon-water vapor mixture inlet temperature of at least 400.degree. C.
According to a preferred further feature of the invention, CO.sub.2 is scrubbed from the methanol synthesis exhaust gas and separated from the used scrubbing liquor and admixed with the hydrocarbons before they are cracked, whereby the C/H ratio is improved and the consumption of thermal-/energy per unit of methanol product is reduced.
According to another preferred feature of the process according to the invention, carbon-containing gaseous constituents are adsorbed from the methanol synthesis exhaust gas and admixed with the hydrocarbons before the latter are cracked, so that the consumption of thermal-/energy per unit of methanol product is reduced.
According to a further preferred feature of the invention, methanol is used to scrub CO.sub.2 from the methanol synthesis exhaust gas.
Within the scope of the invention, the hot synthesis gas is conducted through a convoluted or corrugated tube, which is embedded in the cracking catalyst to transfer heat from the hot synthesis gas to the hydrocarbon-water vapor mixture. A suitable tube element for the furnace is shown in U.S. Pat. No. 3,713,734.
The advantages afforded by the invention reside particularly in a considerable reduction of the heat consumption per ton of pure methanol. The process according to the invention operates in an optimum manner and is highly economical. The capital requirement for a methanol synthesis plant has been much reduced.
The use of the process according to the invention has reduced, the heat requirement per ton of methanol product from the best values known at present and amounting to about 7.85 million kcal/ton to values as low as 7.2 million kcal/ton for example.
In the process, part of the heat content of the hot cracked gases leaving the reaction zone is transferred by means of a tube centrally disposed in the cracking tube to the mixed feedstocks flowing through the catalyst so that the fuel requirement of the tubular heater is much reduced.
Because the cracked gas outlet temperature is lowered from the previous value of 865.degree. C. to about 650.degree. C., the selection of materials for outlet pigtails, outlet manifolds and succeeding high-pressure waste heat boilers is much simplified.
In processes involving the use of light and gaseous hydrocarbon feedstocks, such as natural gas having a C-number of 1.17 and a lower calorific value of 9543 kcal/standard m.sup.3 corresponding to 1015 BTU/SCF, the supply of heat was previously determined by the fuel requirement of the tubular heater and the resulting waste heat content of the flue gas and cracked gas and the steam produced in the synthesizer. Because the heat requirement of the tubular heater could not be decreased, a small part of the supplied energy had to be carried off as steam or as electric energy. The novel system involves a reduced fuel requirement of the tubular heater and enables a virtually perfect energy balance to be achieved in the production of methanol.
The drawing schematically shows a suitable apparatus for effecting catalytic cracking and heat exchange of the product.
The invention will be explained more fully in the following examples.